Field of the Invention
This invention relates to a magnetoresistive sensor, and a device for measuring dimensions including such a sensor. More specifically, the invention relates to the placement of magnetoresistive electrodes in a magnetoresistive sensor.
Electronic devices for measuring length or angular position used in industry, for example, generally have to satisfy several constraints, some of them contradictory. They must provide sufficient precision and resolution, and be able to be used in environments subjected to vibrations and to pollution such as from dust, oil or humidity. Also expected of these sensors is that they can be easily incorporated into apparatus of small volume, without big adjustments or adaptations, while having a high speed of measurement and an electricity consumption as low as possible.
Different types of measuring devices, based on different principles, have been used to fulfill these diverse requirements. In particular, measuring systems using variations in capacity caused by the displacement of a sensor with respect to a scale have been widely used in particular in portable devices such as calipers, for example. These devices must be kept sufficiently clean to function and are thus badly suited to functioning in a humid environment or an environment subjected to splashes of lubricant or oil, for example. Devices for measuring length based on the principle of magnetoresistive electrodes have been proposed, for example in the patent DE 4233331 (IMO), which offer much better resistance to soiling. The device describe din this document comprises a sensor equipped with an array of magnetoresistive electrodes connected so as to define two measuring bridges. The electrodes are disposed and connected in such a way that all the paths are to be found in the same plane, i.e. no crossing of paths is necessary. The sensor is mounted on a slide and can be moved opposite a magnetized scale with a period of magnetization .lambda.. The magnetoresistive electrodes of the sensor are spread over a length equal to two periods of the scale. A displacement of the sensor opposite the scale causes a modification of magnetic field applied to the various magnetoresistive electrodes of the sensor, and thus a modification of their resistance. By putting the measuring bridges under tension, an electric signal is received at their output which is a periodic function of the position of the sensor along the scale.
The two measuring bridges are made up of four magnetoresistive electrodes phase-shifted by .lambda./2. The corresponding electrodes of each bridge occupy positions phase-shifted by .lambda./4. The electrodes of the two bridges are intermixed. This document suggests moreover the use of barber-pole structures. The general theory of barber-pole structures has been explained by F. Dettmann and U. Loreit during the symposium "Magnetoresistive Sensoren, Grundlagen, Herstellung, Anwendung" (Magnetoresistive Sensors, Fundamentals, Production, Application), which was held in Dortmund, Germany, on Jun. 25, 1992. The barber-pole structures are obtained by applying on the magnetoresistive electrodes fine, parallel bands of conducting material inclined by about 45.degree. with respect to the axis of the electrodes. These conducting bands are equipotential lines; between the bands the current takes the shortest path and thus flows with an inclination of 45.degree. with respect to the axis of the magnetoresistive electrodes. This structure allows the direction of vector of current I to be changed. Since the resistance of a magnetoresistive electrode is a function of the angle between the magnetization vector and the current vector, the barber-pole structures allow control of the direction and of the amplitude of the variation in resistance of the electrodes caused by the displacement of the sensor.
Each branch of the measuring bridge is made up of a single magnetoresistive electrode whose width must be sufficient to react to the relatively small magnetic fields generated by the scale. The resistance of the branches of the bridge is thus small, and large currents flow through the measuring bridges. This device therefore has a high electricity consumption.
The patent EP 0 624 778 (Heidenhain) describes a measuring device comprising two bridges of magnetoresistive electrodes, each bridge comprising four branches. Each bridge branch is made up of maximally two magnetoresistive electrodes (see FIG. 3d). The electrical resistance of the branches of the bridge obtainable with this arrangement remains low. This document does not suggest any solution for disposing and connecting magnetoresistive electrodes in such a way as to reduce further the consumption of electricity, so the sensor described is unsuitable for use in an electrically self-powered, portable apparatus, for example in a portable caliper.
The patent U.S. Pat. No. 5,036,276 (Seiko Epson) describes another sensor with magnetoresistive electrodes whose electrodes are connected in such a way as to define four measuring bridges phase-shifted by .pi./4. Each branch of the bridge is formed by a single magnetoresistive electrode. The total resistance of the sensor thus corresponds to that of the four bridges in parallel, with the branches comprising a single electrode, and is particularly low so that the use of the sensor is limited to devices for which the electricity consumption is not crucial.